There are also a whopping number of bug and stability fixes, detailed here.

Contrary to some report that say that the Firefox 5 user interface is identical to that of Firefox 4, there are, in fact, some small changes. Mozilla's Director of Community Development Asa Dotzler details a few changes:

As you can see, the buttons and textfields are much crisper. The corner radius has been sharpened up a bit and the button and textfield shadows have been tightened up and pulled into the border line.

The overall effect might seem subtle but after using it for a while and then going back to Firefox 4, you can really "feel" the difference.

Full release notes here. If you're thinking of running this beta, I suggest you check out the known issues list first.